It is known that exposure of human or animal tissue to ionising radiation will kill the cells thus exposed. This finds application in the treatment of pathological cells. In order to treat tumours deep within the body of the patient, the radiation must however penetrate the healthy tissue in order to irradiate and destroy the pathological cells. In conventional radiation therapy, large volumes of healthy tissue can thus be exposed to harmful doses of radiation, resulting in prolonged recovery periods for the patient. It is, therefore, desirable to design a device for treating a patient with ionising radiation and treatment protocols so as to expose the pathological tissue to a dose of radiation, which will result in the death of these cells, whilst keeping the exposure of healthy tissue to a minimum.
Several methods have previously been employed to achieve the desired pathological cell-destroying exposure whilst keeping the exposure of healthy cells to a minimum. Many methods work by directing radiation at a tumour from a number of directions, either simultaneously from multiple sources or multiple exposures from a single source. The intensity of radiation emanating from each source is therefore less than would be required to destroy cells, but where the radiation beams from the multiple sources converge, the intensity of radiation is sufficient to deliver a therapeutic dose.
The point of intersection of the multiple radiation beams is herein referred to as the “target point”. The radiation field surrounding a target point is herein referred to as the “target volume”, the size of which can be varied by varying the size of the intersecting beams.
A radiation device of this type is sold by the applicant as the Leksell Gamma Knife® (LGK). The LGK device is described in U.S. Pat. No. 4,780,898 and U.S. Pat. No. 5,528,651. In the LGK, a plurality of radiation sources are distributed around the head of the patient, in a hemispherical arrangement. By means of suitable collimators, the radiation beams from each source are focussed to a small volume in the brain. The LGK is commonly regarded as the ‘gold standard’ for delivering radiation to destroy pathological tissues in the brain, as a result of (i) the low background radiation away from the target volume as compared to the high radiation intensity within the target volume and (ii) the small dimensions of the target volume. This enables the surgeon to excise small areas accurately and swiftly, without damage to surrounding structures. An acknowledgement of the LGK appears at Nakagawa et al, Radiation Medicine, Vol 21, No. 4, pp 178-182, 2003.
The LGK uses Magnetic Resonance Imaging (MRI), Computer Tomography (CT), PET and/or angiography to determine the exact location of the tumour, with the patient being held in a fixed position by the use of a reference frame, to construct a three-dimensional image of the target. The treatment parameters for each radiation beam are then determined such that the pathological tissue is treated to the necessary dose of radiation, whilst surrounding healthy tissue receives a minimal dose of radiation.
The treatment may be spread over a number of days or weeks, thus requiring that the patient is placed in exactly the same position in relation to the point of intersection of the converging beams at each treatment, to avoid the risk that pathological tissue is missed or that surrounding healthy tissue is irradiated unintentionally. This is extremely important in the case where diseases in the brain are treated, which requires the radiation beam to be focussed with pinpoint accuracy to avoid damage to sensitive areas such as e.g. the optic nerve, which if irradiated will result in the patient losing their sight, even with only small doses. This method therefore calls for the presence of a highly skilled, specialist team of technical experts to provide radiation treatment using these appliances.
A modification of the LGK has been proposed in the form of U.S. Pat. No. 5,757,886 (Song), which involves placing cobalt sources in a ring configuration. A group of different collimators for each source are mounted on a hemispherical support that can be rotated relative to the sources to bring one collimator of the group into register, for each source. This allows a wider choice of collimators, at the expense of fewer cobalt sources and correspondingly greater treatment times.
Other forms of radiotherapy are delivered using linear-accelerator-based systems. A linear accelerator uses radio-frequency energy to create a varying magnetic & electrical field in a elongate accelerating chamber—hence a “linear” accelerator. Electrons are fed into the chamber and are accelerated to near light speed. The resulting beam can be used directly as a form of radiation, but it is more usual to direct this to a suitable “target”, a block of an appropriate heavy metal such as tungsten. The electron beam impinges on the tungsten block and causes it to emit a beam of x-radiation. The geometry of the electron beam and the tungsten surface are arranged so that the x-ray beam departs perpendicular to the incoming electron beam and can thus be directed towards a patient.
The x-ray beam is collimated to a suitable shape and passes through the patient causing tissue damage. By suitable collimation and by moving the linear accelerator around the patient so that it approaches from a range of directions, such systems can minimise the dosage outside the tumour and maximise it within the tumour.
The principal disadvantage with linear accelerator systems is that the accelerator is extremely heavy. To combine the necessary electrical and thermal properties requires the accelerator chamber to be constructed of large copper blocks. The production of x-rays also produces unwanted radiation, which has to be attenuated by large amounts of shielding material e.g. Tungsten, and this combined with the other components required to operate the linear accelerator will cause the apparatus as a whole to be extremely heavy.
This weight must be supported, and the apparatus moved accurately so that the radiation beam can be directed towards the patient from a range of directions. For bodily tumours, the usual compromise is to mount the linear accelerator in an arm extending from a rotateable mount. The beam then exits from the end of the arm, directed inwardly towards the centreline of the mount. A patient supported at the intersection of the centreline and the beam can them be treated; as the mount rotates, the beam will meet the patient from a range of directions within the same plane.
Such systems are not generally used for tumours of the brain. They are too inflexible, as the beam must approach the patient from a direction that is within a single plane. If that plane includes a sensitive structure, such as the optic nerve, severe damage could be caused. In the LGK, for example, beams approach from all directions and the element that would interfere with such a structure can be blocked.
It is possible to mount a linear accelerator on a robotic arm, to allow a wide range of possible motions. Proposals of this type have been made, and these would, in theory, overcome this problem. However, the great weight of the linear accelerator structure means that it is extremely difficult to engineer such a robotic arm so that the movement is carried out with the precision required for tumours of the brain. Such tumours require placement accuracy of tens of thousandths of a inch or less, and to move an item weighing several tons at the end of an arm that may be several yards long to such levels of accuracy is a near impossible task. Thus, whilst such designs can be constructed and find application to bodily tumours, they are not sufficiently accurate for use with tumours of the brain.
Nakagawa et al, cited above, proposes a system of this type in which some flexibility of movement is sacrificed in favour of greater accuracy. The linear accelerator is mounted on one end of a C-arm, which is (in turn) held in a rotateable support. The C-arm can move on its support; thus at its two extremities of motion it resembles more a U-arm or an inverted U. As it moves, the angle of entry of the radiation beam will change. Thus, combined with rotation of the support, will give the necessary range of motion. However, as the C-arm moves, the centre of gravity of the apparatus will shift, causing errors. To counteract this, Nakagawa et al require a complex system of retractable balance weights in order to prevent movement; this is a potential weakness in the accuracy of the apparatus.